Conventionally, a hydraulic pump driven by an engine and a hydraulic motor driven by oil are often used in a construction machine and the like.
For example, an axial swash plate hydraulic pump/motor is provided with a rotational shaft rotatably attached in a casing, a cylinder block rotating together with the rotational shaft, a plurality of pistons fittedly inserted into a plurality of cylinder holes formed on the cylinder block so as to be able to reciprocate, a swash plate provided in the casing so as to be tilted relative to the rotational shaft for supporting tip ends of the pistons so as to be able to slidingly contact, and a valve plate slidingly contacting a rear end face of the cylinder block, and is configured to distribute oil in the cylinder holes through a port provided on the valve plate.
When using the swash plate hydraulic pump/motor as the hydraulic pump, the cylinder block is rotated by rotate-driving the rotational shaft by the engine and the like and the piston is allowed to reciprocate, thereby pressurizing the oil sucked from a low-pressure side port to the cylinder hole by the piston to discharge from a high-pressure side port.
Also, when using the swash plate hydraulic pump/motor as the hydraulic motor, the oil is supplied from the high-pressure side port to the cylinder hole and the piston is protruded from the cylinder hole to press the swash plate, thereby rotating the rotational shaft together with the cylinder block.
As such swash plate hydraulic pump/motor, the one provided with a rotation sensor for detecting a rotational speed of the cylinder block is known (refer to the Patent Document 1). FIG. 7 is a cross-sectional view illustrating a schematic configuration of the swash plate hydraulic pump/motor disclosed in the patent document 1. A swash plate hydraulic pump/motor 100 is provided with a casing 110, a lid body 120, a rotational shaft 130, a cylinder block 140, a piston 150, a valve plate 160 and a swash plate 170. Detected concave portions 520 are formed on an outer circumferential surface of the cylinder block 140 at predetermined intervals. Electromagnetic pick up rotation sensors 500 for detecting the detected concave portions 520 is arranged on a position opposed to the detected concave portions 520 and is fixed to the casing 110. When the cylinder block 140 rotates, each detected concave portion 520 passes through the position of the rotation sensor 500, thereby periodically changing distance (magnetic field) between the rotation sensor 500 and the detected concave portions 520. The rotation sensor 500 transmits a detection signal corresponding to change in the magnetic field to a controller not illustrated. The controller shapes an alternating-current waveform of the detection signal from the rotation sensor 500 and calculates a frequency thereof as a rotational speed of the cylinder block 140.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-267679